1,720,953 research outputs found
Infrasound and the Dynamical Stratosphere: A new application for operational weather and climate prediction
No full textInfrasound is low-frequency inaudible sound (< 20Hz), used as a waveform verification technique for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). In this thesis, it is shown that infrasound can provide useful additional upper atmospheric observations in a region where data coverage is sparse; beyond the middle stratosphere direct information of wind is missing in current numerical weather prediction models. Passive ambient infrasound signals can act as an atmospheric probe. Observed and simulated infrasound wavefront parameters are compared, i.e., back azimuth, apparent velocity, or travel time. Wavefront parameters are extracted from the continuous waveform recordings using signal detectors and array processing techniques and simulated by ray theory using various atmospheric conditions. The theoretical basis of this method relies on the assertion that sound propagates through a particular atmospheric state. The state that is closest to reality will then lead to simulated values that are closest to the observed values. It is demonstrated that infrasound has the potential to contribute to operational weather prediction applications by validation of atmospheric analysis and forecast products, in particular in regions above 30 km altitude. In addition, knowledge of the dynamical stratosphere is utilized for infrasound monitoring, in order to benefit CTBT verification efforts.Applied Geophysics and Petrophysic
A bird's-eye view on infrasound: High-resolution methods to unravel the ambient microbarom wavefield
No full textApplied Geophysics and Petrophysic
Hydroacoustic Travel Time Variations as a Proxy for Passive Deep-Ocean Thermometry: A Cookbook
No full textWe report on the extraction of deep ocean travel time variations from time-lapse cross-correlations between a hydrophone station and a three-component broadband seismometer. The signals we cross-correlate in this study result from repeated activity by the Monowai seamount, one of the most active submarine volcanoes of the Tonga-Kermadec ridge. In particular, we introduce a specific workflow to exploit repetitive hydroacoustic underwater source activity, which we detail to such an extent that it serves as an example (or “cookbook”). For this reason, we have made the source code publicly available. The workflow proposed in this study (a) overcomes differences in instrument sensitivity and sample rates, (b) involves the selection of eligible cross-correlations based on a source activity criterium as well as slowness analysis, and (c) extracts the travel time variations in distinct frequency bands. In our case, the two frequency bands are 3–6 and 6–12 Hz. We find that the estimated travel time variations in both frequency bands consist of a complex periodic pattern superimposed on a robust linear trend. This linear trend is decreasing, which we attribute to increasing water temperatures along the propagation path of the hydroacoustic signals.Applied Geophysics and Petrophysic
Probabilistic infrasound propagation using realistic atmospheric perturbations
No full textThis study demonstrates probabilistic infrasound propagation modeling using realistic perturbations. The ensembles of perturbed analyses, provided by the European Centre for Medium-Range Weather Forecasts (ECMWF), include error variances of both model and assimilated observations. Ensemble spread profiles indicate a yearly mean effective sound speed variation of up to 8 ms?1 in the stratosphere, exceeding occasionally 25 ms?1 for a single ensemble set. It is shown that errors in point estimates of effective sound speed are dominated by variations in wind strength and direction. One year of large mining explosions in the Aitik mine, northern Sweden, observed at infrasound array IS37 in northern Norway are simulated using 3-D ray tracing. Probabilistic propagation modeling using the ensembles demonstrates that small-scale fluctuations are not always necessary to improve the match between predictions and observations.Geoscience & EngineeringCivil Engineering and Geoscience
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
The life cycle of a sudden stratospheric warming from infrasonic ambient noise observations
No full textA method is presented to study the life cycle of a SSW using infrasonic ambient noise observations. The potential of infrasound is shown to provide the missing observations required by numerical weather prediction to better resolve the upper atmosphere. The 2009 major SSW is reanalyzed using the Evers and Siegmund (2009) data set. Microbarom observations are evaluated to identify detections that cannot be explained by the analysis of the European Centre for Medium-Range Weather Forecasts. Identified differences can be related to either the altitude limit of the analysis, not resolving thermospheric ducts, or to an actual error in the analysis. Therefore, a first-order model is used to relate observations with the analysis, existing of the Waxler et al. (2007) microbarom source model, including bathymetry to allow column resonances, and an atmospheric propagation model using 3-D ray tracing. Daily normalized spectral powers are proposed to distinguish stratospheric from thermospheric return height, based on the different signature of solar tidal amplitude fluctuations. It is shown that a SSW is not a smooth event as following from the analysis but a series of abrupt changes with a period of 10 to 16 days, increasing in intensity and duration. This is in agreement with the wave period of Rossby waves, interacting with the stratospheric circumpolar vortex. The type of vortex disturbance, split or reversal, can be deduced from the combined effect of the change in back-azimuth direction, solar tidal signature type, and/or phase variation of the amplitude variation of the observed microbaroms.Geoscience & EngineeringCivil Engineering and Geoscience
3D array processing forinfrasound arrays withnon-ground based detectors
No full textArray processing of infrasound data uses the coherency between sensors to calculate directions of incoming waves. The Cabauw Infrasound Array (CIA) provides unique opportunities for infrasound processing. Because unlike most infrasound arrays, CIA has sensors with height differences. Specifically sensors distributed on a 200m tall tower. But the data from these tower sensors are of limited use for time-domain array processing due to the measurement of both the incoming wave and its reflection. To make the measurements suitable for array processing the reflection needs to be removed. The ground measurements are used to approximate the waveform and a time-shifted ground average is removed from the tower measurements. The new cleaned data was successfully used for full 3D array processing.Applied Geophysics | IDEA Leagu
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